Efficient utilization of lumber production requires that the material be graded according to its intended use. In this way an effective and economic match can be made between the lumber needs of end-users and the lumber supply of the producer. Many factors control the suitability of lumber for any particular purpose. They include the degree of straightness, the amount of any wane, and the presence and size of knots, splits, shakes, etc. These and other factors are currently assessed by trained personnel using established visual grade rules.
Two engineering properties are often of great importance when designing wood structures. They are wood strength and wood stiffness. In the visual system of grading, these properties are established from values measured during destructive tests on extensive samples of each visual grade. However, the process of visual grading is not very effective at identifying wood strength and stiffness. Thus, a sample of material of a given visual grade contains pieces whose strengths and stiffnesses vary over very wide ranges. For example, the strength of the strongest piece in a batch of a given visual grade is typically 5-10 times that of the weakest piece. Thus, for safe design, the strength and stiffness of the worst likely piece has to be assumed. This is clearly very wasteful of the majority of superior pieces which are being used at well below their actual capacities.
Such waste can be reduced by developing and using techniques which better identify the superior pieces and reliably distinguish them from the inferior pieces. This process is called "Machine Stress Rating" or "MSR". A method which has been developed for this purpose involves measuring lumber stiffness in the plank bending mode. For example, a machine which is commonly used for this purpose is the "Continuous Lumber Tester", made by Metriguard Inc., of Pullman, Wash. This machine measures plank stiffness by moving the wood longitudinally through a series of instrumented rollers. These rollers are slightly displaced from a straight line so as to bend the wood by predetermined amounts, first in one direction and then in the other. The machine then infers the wood plank stiffness from the bending forces measured by the instrumented rollers.
Plank bending stiffness is an excellent indicator of the stiffness in the joist bending mode, which is the most common design configuration. The plank stiffness is also a modest indicator of tensile and bending strength. Using measured statistical relationships between plank stiffness and strength, and also visual over-rides for knots and other defects, lumber is segregated into various strength grades.
The machine stress rating process based on plank stiffness measurements is an improvement over visual grading as an indicator of wood strength. However, it is far from ideal because the range of strengths in each MSR grade remains quite large. Thus, most pieces must still be used at well below their capacity. Additional wastage of material strength capacity occurs because the imprecise grading method often incorrectly assigns strong material to low strength grades.
Another difficulty with wood strength grading by plank stiffness measurement is that typical machines for measuring stiffness are physically large, mechanically complex, and costly to maintain. Also, they are insensitive to the stiffness of the material close to the ends of the lumber. Thus, several feet of each board remain ungraded.
This invention describes a more precise and reliable method of wood strength grading based on measurements of wood density (specific gravity) profile. It achieves a superior level of precision by more accurately identifying the features which control wood strength. The procedure also greatly enhances the engineering performance of the lumber when it is specified according to a reliability based design format. An additional advantage of this invention over the plank stiffness method is that the hardware is compact and easy to maintain. It makes non-contacting measurements over the entire length of the wood, right up to the two ends. The density profile method can also make modestly precise estimates of lumber stiffness, of similar statistical precision to the strength estimates available from measurements of plank stiffness. These stiffness estimates are inferior to those available by direct stiffness measurement, and form only a subsidiary objective of this invention.